Refrigeration systems are often operated by adjusting a degree of opening of an expansion valve and/or by opening/closing an expansion valve, thereby controlling the amount of liquid refrigerant being supplied to an evaporator. It is desirable to control the expansion valve in such a manner that it is obtained that all of the liquid refrigerant which is supplied to the evaporator is evaporated before exiting the evaporator, and in such a manner that mixed phase refrigerant is present at or immediately before the outlet of the evaporator. In the case that liquid refrigerant is allowed to exit the evaporator there is a risk that this liquid refrigerant reaches the compressor, and this may in some cases cause damage to the compressor. On the other hand, in the case that the liquid refrigerant evaporates while passing the first part of the evaporator, then the refrigeration capacity of the evaporator is not utilised to the full extent.
The superheat of the refrigerant provides information as to whether or not the situation described above has been obtained. Superheat is normally defined as the difference between the actual temperature of a fluid and the boiling point of the fluid. Accordingly, the superheat depends on the temperature as well as the pressure of the fluid. Thus, the superheat is a suitable parameter for controlling the opening degree of the expansion valve. It is normally desired that the refrigerant has a low, but positive, superheat. When this is the case, the situation described above is obtained, i.e. the refrigeration capacity of the evaporator is utilised to the greatest possible extent, and the risk of causing damage to the compressor due to liquid refrigerant being passed through the evaporator is minimised.
Thus, the superheat value of the refrigerant leaving the evaporator should, ideally, be zero. However, when the superheat value is zero it is impossible to know whether the evaporator is operating in an optimal manner, i.e. the liquid refrigerant is exactly evaporated in the evaporator, or whether a large amount of liquid refrigerant is passing through the evaporator. Therefore, in most prior art refrigeration systems the opening degree of the expansion valve is controlled in such a manner a positive superheat, e.g. of approximately 5-10 K, is maintained. Thereby it is ensured that no liquid refrigerant is allowed to pass through the evaporator. However, this has the consequence that the refrigeration capacity of the evaporator is not fully utilised.
In A. Tambovtsev and H. Quack, “COP Improvements by Transfer of the Superheating into the Internal Heat Exchanger”, ICR07-B2-1406, describes a refrigeration system and a method of controlling a refrigeration system. The refrigeration system has been provided with an additional internal heat exchanger which ensures that liquid refrigerant which has been allowed to pass through the evaporator is evaporated before it reaches the compressor. Thereby a slightly wet outlet is obtained, and the entire heat transfer area of the evaporator can, thus, be used for evaporating refrigerant, and the efficiency of the evaporator is improved. It is, however, a disadvantage that the solution suggested in this paper requires an additional heat exchanger, since this increases the costs of producing the refrigeration system, as well as the complexity of the system.